In wireless communication networks, there is always a challenge to obtain good performance and capacity for a given communications protocol, its parameters and the physical environment in which the wireless communication network is deployed.
According to the LTE (Long Term Evolution) telecommunications standard cell-specific reference signals (CRS) are transmitted in all downlink subframes. In addition to assisting downlink channel estimation, the CRS are also used for mobility measurements performed by the wireless devices (in LTE also known as user equipment, UE). The CRS are generally intended for use by all the UEs in the coverage area of the network node transmitting the downlink signals. As of LTE Release-10, specific reference signals are provided for measuring the channel for the purpose of generating channel state information (CSI) feedback from the UE. The latter reference signals are referred to as CSI-RS. CSI-RS are not transmitted in every subframe, and they are generally sparser in time and frequency than reference signals used for demodulation. CSI-RS transmissions may take place every fifth, tenth, twentieth, fortieth, or eightieth subframe, as determined by a periodicity parameter and a subframe offset, each of which are configured by Radio Resource Control (RRC) signalling.
A UE operating in a connected mode can be requested by the network node (base station) to perform channel state information (CSI) reporting. This reporting may comprise, for example, reporting a suitable rank indicator (RI) and one or more precoding matrix indices (PMIs), given the observed channel conditions, as well as a channel quality indicator (CQI). Other types of CSI are also conceivable, including explicit channel feedback and interference covariance feedback. The CSI feedback assists the network node in scheduling, including deciding which subframe and resource blocks to use for the transmission, as well as deciding which transmission scheme and/or precoder should be used. The CSI feedback also provides information that can be used to determine a proper user bit-rate for the transmission, i.e., for link adaptation.
In order to support mobility, a wireless device needs to continuously search for, synchronize to, and estimate the reception quality of both its serving cell and neighbour cells (i.e. cells neighbouring the serving cell). The reception quality of the neighbour cells, in relation to the reception quality of the current cell (i.e. the serving cell), is then evaluated in order to determine whether a handover, for UEs in the connected mode, or cell re-selection, for UEs in an idle mode, should be carried out. For wireless devices in connected mode, the handover decision is taken by the network, based on measurement reports provided by the wireless devices. Examples of such reports are reference signal received power (RSRP) and reference signal received quality (RSRQ).
Typically, RSRP measurements are accomplished through wireless device measurements on downlink reference signals (RS) and feedback of such measurements to the network. However, there may be a number of issues associated with the downlink based approach. For example, the network may need to wait for an updated measurement from the wireless device, which typically is performed sparsely in time. For example, in case of dense deployments and/or intense traffic, the uplink (UL) signalling in the network due to downlink measurements may be undesirably high. For example, in case of dense deployments, it may be challenging for the network to identify which RS a wireless device should measure on. For example, some nodes associated with low transmission power may not be able to reach a wireless device for downlink measurements, even though such network nodes are of potential interest as reception points. The same issue occurs for network nodes that are not provided with a transmitter on the carrier of interest. In general terms, a carrier signal as herein defined is the sum of a number of orthogonal sub-carriers, where baseband data on each sub-carrier is independently modulated. For example, for certain deployments, cell specific reference signals suitable for RSRP measurements may not be available, at least for certain carriers optimized for data transmission.
Uplink RSRP measurements in principle may be possible in LTE, e.g., based on sounding reference signals (SRS). However, SRS were originally designed with the aim of link adaptation and may not necessarily be optimized for long range RSRP measurements. In particular, SRS are designed for channel estimation (i.e., for estimating the time or frequency domain response of the channel) while in case of RSRP measurements only the received power is of interest. Furthermore, in order to allow for link adaptation, the transmission power for SRS is linked to the transmission power on the physical uplink shared channel (PUSCH) by a configurable offset (with the exception of power limited transmission). To allow for RSRP measurements to both the serving cell and neighbour cells, a large number of wireless devices may require multiplexed transmission of the SRS. However, the multiplexing capacity for SRS, i.e. the number of SRS that are orthogonal to each other and may be transmitted in parallel in the same subframe, may not be sufficient for supporting uplink measurements from a large number of wireless devices.
Hence, there is still a need for an improved transmission and reception of reference signals.